Many Techmarines are equipped with a powerful servo-arm that can be used to carry out hasty battlefield repairs. The servo-arm counts as a power fist in close combat. It always hits on a 4+ and is rolled separately from the Techmarine's own Attacks. In addition it may be used at the start of any Space Marine turn to repair an immobilised vehicle that is in base contact with the Techmarine. On a D6 roll of a 6 the vehicle is repaired and may move normally.
Servo-Arm
Source: Codex: Space Marines, p. 35